I just have to know: how many books are in the series Star Trek: Starfleet Corps of Engineers (ST:SCE). On-line is a option if you want to include those.

There are 87

Here's a list.

The Belly of the Beast by Dean Wesley Smith 1

Fatal Error by Keith R. A. DeCandido 2

Hard Crash by Christie Golden 3

Interphase, Part One of Two by Dayton Ward 4

Have Tech, Will Travel by Keith R. A. DeCandido omnibus 1-4

Interphase, Part Two of Two by Dayton Ward 5

Cold Fusion by Keith R. A. DeCandido 6

Invincible, Part One of Two by David Mack 7

Invincible, Part Two of Two by David Mack 8

Miracle Workers by Keith R. A. DeCandido omnibus 5-8

The Riddled Post by Aaron Rosenberg 9

Here There Be Monsters by Keith R. A. DeCandido 10

Ambush by Dave Galanter 11

Some Assembly Required by Scott Ciencin 12

Some Assembly Required [omnibus] by Greg Brodeur omnibus 9-12

No Surrender by Jeff Mariotte 13

Caveat Emptor by Ian Edginton 14

Past Life by Robert Greenberger 15

Oaths by Glenn Hauman 16

No Surrender by Mike Collins omnibus 13-16

Foundations, Book One of Three by Dayton Ward 17

Foundations, Book Two of Three by Dayton Ward 18

Foundations, Book Three of Three by Dayton Ward 19

Foundations by Dayton Ward omnibus 17-19

Enigma Ship by J. Steven York 20

War Stories, Book One of Two by Keith R. A. DeCandido 21

War Stories, Book Two of Two by Keith R. A. DeCandido 22

Wildfire Book 1 (Star Trek: Starfleet Corps of Engineers 23) by David Mack 23

Wildfire Book 2 by David Mack 24

Wildfire by Keith R. A. DeCandido omnibus 20-24

Home Fires by Dayton Ward 25

Age of Unreason by Scott Ciencin 26

Balance of Nature by Heather Jarman 27

Breakdowns by Keith R. A. DeCandido 28

Breakdowns by Scott Ciencin omnibus 25-28

Aftermath by Christopher L. Bennett 29

Ishtar Rising Book 1 by Michael A. Martin 30

Ishtar Rising Book 2 by Michael A. Martin 31

Buying Time by Robert Greenberger 32

Collective Hindsight Book 1 by Aaron Rosenberg 33

Collective Hindsight Book 2 by Aaron Rosenberg 34

The Demon Book 1 by Loren Coleman 35

The Demon Book 2 by Loren Coleman 36

Aftermath by Christopher L. Bennett omnibus 29-36

Ring Around the Sky by Allyn Gibson 37

Orphans by Kevin Killiany 38

Grand Designs (novella) by Dayton Ward 39

Failsafe by David Mack 40

Bitter Medicine by Dave Galanter 41

Sargasso Sector by Paul Kupperberg 42

Grand Designs by Dayton Ward omnibus 37-42

Paradise Interrupted by John S. Drew 43

Where Time Stands Still by Dayton Ward 44

The Art of the Deal by Glenn Greenberg 45

Spin by J. Steven York 46

Creative Couplings, Part 1 by Glenn Hauman 47

Creative Couplings, Part 2 by Glenn Hauman 48

Small World by David Mack 49

Creative Couplings by David Mack omnibus 43-49

Malefictorum by Terri Osborne 50

Lost Time by Ilsa J. Bick 51

Identity Crisis by John J. Ordover 52

Fables of the Prime Directive by Cory Rushton 53

Security by Keith R. A. DeCandido 54

Wounds, Book 1 by Ilsa J. Bick 55

Wounds, Book 2 by Ilsa J. Bick 56

Wounds by Ilsa J. Bick omnibus 50-56

Out of the Cocoon by William Leisner 57

Star Trek: Honor (Star Trek: Starfleet Corps of Engineers Book 58) by Kevin Killiany 58

Blackout by Phaedra M. Weldon 59

The Cleanup by Robert T. Jeschonek 60

Out of the Cocoon by William Leisner omnibus 57-60

Progress by Terri Osborne 61

The Future Begins by Michael Schuster 62

Echoes of Coventry by Richard C. White 63

Distant Early Warning by Dayton Ward 64

10 is Better Than 01 by Heather Jarman 65

Many Splendors by Keith R. A. DeCandido 66

What's Past by Terri Osborne omnibus 61-66

Turn the Page by Dayton Ward COE #1

Troubleshooting by Robert Greenberger COE #2

The Light by Jeff D. Jacques COE #3

The Art of the Comeback by Glenn Greenberg COE #4

Signs from Heaven by Phaedra M. Weldon COE #5

Ghost by Ilsa J. Bick COE #6

Remembrance of Things Past, Book I by Terri Osborne COE #7

Remembrance of Things Past: Book II by Terri Osborne COE #8

Star Trek: Starfleet Corps of Engineers: What's Past

Star Trek: Starfleet Corps of Engineers: Foundations

Star Trek: Starfleet Corps of Engineers: Interphase

Star Trek: Starfleet Corps of Engineers: Collective Hindsight

Star Trek: Starfleet Corps of Engineers: The Demon

Star Trek: Starfleet Corps of Engineers: Ishtar Rising
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Worth noting: this list includes 13 Omnibus editions which bring the list of actual works down to 74.